DEM Generation with digital photogrammetry and laser scanning in architectural structures survey

DEM generation in architecture presents well known problems when the structure is complex and has irregular and discontinue surfaces. Data acquired from laser scanner and photogrammetric systems can be employed to best represent different aspects of the same object: every survey technique is useful to register (and then to show, after data elaboration) some kind of information due to the specific capability to “read” the object surface. Photogrammetry is able to provide, according to a previous survey project, different kinds of plotting, with an expected accuracy: - description of the object (points, lines) and main discontinuities (break lines); - digital elevation model (DEM) with a forecast resolution of surfaces bounded by edges. In this work we match and compare data collected by laser scanner and by digital photogrammetry and investigate their skills and weakness. Until now automatic techniques for feature extraction do not give satisfactory results in architectonic field, while DEM generation has been strongly improved by automatism developed in digital photogrammetry software. However our experience shows that DEM generated with automatic image correlation does not give a complete representation of the stereoscopic model. Poor quality images, as well as non uniform texture on the surface, should cause important lacks. On the other hand, laser scanner allows to acquire DEM with planned step and extension, closely related to the acquisition field conformation and the object features. In both cases, automatic DEM generation is not always satisfactory: unavoidable interpolation occurring when triangulation transforms points clouds into surfaces (TIN) causes typical edge rounding effects. Sometimes the 3D model could be good from the metric point of view (differences between 3D model and object are in the predefined range) but a visual inspection could be unsatisfactory. We analyzed and compared surfaces made by the same triangulation algorithm, starting from both photogrammetric and laser scanner DEM.